1. Field of the Invention
This invention relates to a process for selectively producing olefins and aromatic hydrocarbons (hereinafter abbreviated as BTX) by thermal cracking of hydrocarbons. More particularly, it relates to a process for producing olefins and BTX in high yield and high selectivity which comprises the steps of burning hydrocarbons with oxygen in the presence of steam to generate a hot gas comprising steam, and feeding, to the hot gas comprising steam and serving as a heat source for thermal cracking, different types of hydrocarbons from feeding positions enabling the respective hydrocarbons to be thermally cracked under optimum cracking conditions in view of their cracking characteristics.
2. Description of the Prior Art
As is well known, the tubular-type thermal cracking process called steam cracking has heretofore been used to convert, into olefins, light gaseous hydrocarbons such as ethane and propane as well as liquid hydrocarbons such as naphtha and kerosine. According to this process, heat is supplied from outside through tube walls, thus placing limits on the heat per rate speed and the reaction temperature. Ordinary conditions adopted for the process include a temperature below 850.degree. C. and a residence time ranging from 0.1 to 0.5 second. Another process has been proposed in which use is made of small-diameter tubes so that the cracking severity is increased in order to effect the cracking within a short residence time. In this process, however, because of the small inner diameter, the effective inner diameter is reduced within a short time owing to coking on the inner walls. As a consequence, the pressure loss in the reaction tube increases with an increasing partial pressure of hydrocarbons, thus worsening the selectivity to ethylene. This, in turn, requires short time intervals of decoking, leading to the vital disadvantage that because of the lowering in working ratio of the cracking furnace and the increase of heat cycle due to the decoking, the apparatus is apt to damage. In the event that the super high temperature and short time cracking would become possible, it would be difficult to stop the reaction, by quenching, within a short time corresponding to the cracking severity. This would result in the fact that the selectivity to ethylene which has once been established in a reactor unit considerably lowers by shortage of the quenching capability of a quencher. In view of these limitations on the apparatus and reaction conditions, starting materials usable in the process will, at most, cover gas oils. Application to heavy hydrocarbons such as residues, cannot be expected. This is because high temperature and long time reactions involve side reactions of polycondensation with coking occurring vigorously and a desired gasification rate (ratio by weight of a value obtained by subtracting an amount of C.sub.5 and heavier hydrocarbons except for BTX from an amount of hydrocarbons fed to a reaction zone, to an amount of starting hydrocarbon feed) cannot be achieved. Consequently, the yield of useful components lowers. Once a starting material is selected, specific cracking conditions and a specific type of apparatus are essentially required for the single starting material and a product derived therefrom. This is disadvantageously unadaptable to the type of starting material and the selectivity to product.
For instance, a currently used, typical tubular-type cracking furnace has the central aim in the production of ethylene. Thus, it is difficult to arbitrarily vary yields of other by-products such as propylene, C.sub.4 fractions and BTX in accordance with a demand and supply balance. This means that since it is intended to secure the production of ethylene from naphtha as will otherwise be achieved in high yield by high severity cracking of other substitute materials, great potentialities of naphtha itself for formation of propylene, C.sub.4 fractions such as butadiene, and BTX products are sacrificed. The thermal cracking reaction has usually such a balance sheet that an increase in yield of ethylene results in an inevitable reduction in yield of propylene and C.sub.4 fractions.
Seveal processes have been proposed in order to mitigate the limitations on both starting materials and products. In one such process, liquid hydrocarbons such as petroleum are burnt to give a hot gas. The hot gas is used to thermally crack hydrocarbons under a pressure of from 5 to 70 bars at a reaction temperature of from 1,315.degree. to 1,375.degree. C. for a residence time of from 3 to 10 milliseconds. In the process, an inert gas such as CO.sub.2 or N.sub.2 is fed in the form of a film from the buring zone of the hot gas toward the reaction zone so as to suppress coking and make it possible to crack heavy oils such as residual oils.
Another process comprises the steps of partially burning hydrogen to give hot hydrogen gas, and thermally cracking various hydrocarbons such as heavy oils an atmosphere of hydrogen under conditions of a reaction temperature of from 800.degree. to 1800.degree. C., a residence time of from 1 to 10 milliseconds and a pressure of from 7 to 70 bars thereby producing olefins. The thermal cracking in an atmosphere of great excess hydrogen enables one to heat hydrocarbons rapidly and crack within a super-short residence time. Likewise, suppression of coking enables one to effect cracking of heavy oils. However, power consumptions for recycle and separation of hydrogen, make-up, and energy for pre-heating place an excessive economical burden on the process.
All the processes require very severe reaction conditions in order to obtain olefins in high yields from heavy hydrocarbons. As a result, olefinic products obtained are predominantly composed of C.sub.2 products such as ethylene, acetylene and the like, with an attendant problem that it is difficult to obtain propylene, C.sub.4 fractions, and BTX at the same time in high yields.
A further process comprises separating a reactor into two sections, feeding a paraffinic hydrocarbon of a relatively small molecular weight to an upstream, high temperature side so that it is thermally cracked at a relativey high severity (e.g. a cracking temperature exceeding 815.degree. C., a residence time of from 20 to 150 milliseconds), thereby improving the selectivity to ethylene, and feeding gas oil fractions to a downstream, low temperature side so as to thermally crack them at a low severity for a long residence time, e.g. a cracking temperature below 815.degree. C. and a residence time of from 150 to 2,000 milliseconds whereby coking is suppressed. Instead, the gasification rate is sacrificed. Similar to the high temperature side, the purposes at the low temperature side are to improve the selectivity to ethylene.
In the above process, the starting materials are so selected as to improve the selectivity to ethylene: paraffinic materials which are relatively easy for cracking are fed to the high temperature zone and starting materials abundant with aromatic materials which are relatively difficult to crack are fed to the low temperature zone.
However, starting materials containing aromatic components are cracked in the low temperature reaction zone at a low severity, so that components which can be evaluated as valuable products when gasified are utilized only as fuel. Thus, this process is designed to place limitations on the types of starting materials and products, thus presenting the problem that free selection of starting materials and production of intended products are not possible.
We have made intensive studies to develop a thermal cracking process of hydrocarbons to selectively obtain desired types of olefins and BTX in high yields from a wide variety of hydrocarbons ranging from light to heavy hydrocarbons in one reactor while suppressing the coking. As a result, it has been found that thermal cracking of hydrocarbons effectively proceeds by a procedure which comprises the steps of burning hydrocarbons with oxygen in the presence of steam to produce a hot gas stream containing steam, feeding arbitrary starting materials to different cracking positions in consideration of the selectivity to desired products and the characteristics of the starting hydrocarbons. By the thermal cracking, a variety of hydrocarbons ranging from gas oils such as light gas and naphtha to heavy oils such as asphalt can be treated simultaneously in one reactor. Moreover, olefins and BTX can be produced in higher yields and higher selectivity than in the case where individual hydrocarbons are thermally cracked singly as in a conventional manner. The present invention is accomplished based on the above finding.